Laser-based machining is indispensable today in many areas of industrial production. The machining quality achieved by the use of lasers cannot be obtained in many cases by alternative methods. On top of this, high processing speeds are realized in automated production, allowing economically advantageous application of laser-based methods. This can be effected by a multiplicity of complex devices and systems, which are adapted as workstations for a number of specific machining tasks. Industrial workstations comprise, for example, systems for cutting metal sheets as well as devices for machining structures on a microchip. There are also laser systems for carrying out medical processes, for example for correction of eyesight in the case of myopia or hyperopia. These laser systems effect machining of human tissue, while in industrial applications solid materials, such as steel or ceramics, are machined in most cases. Applications for liquid materials, for example laser-assisted polymerization in rapid prototyping, are also important.
If an object is irradiated with laser radiation, this is effected in most cases in order to effect a measurement or in order to machine the material of the object. In each case, there is an interaction between the laser radiation and the material, for example a reflection, a scattering process or an absorption. Important applications in this context are interferometric measuring methods, conventional laser microscopy or exposure of photolacquers in semi-conductor lithography.
However, there are also applications in which conventional, i.e. linear interaction, between the laser radiation and the material is not possible, for example because the material is transparent. In such applications, non-linear interaction between the laser radiation and the material can be utilized for machining, i.e. in particular non-linear absorption of the laser radiation by the material of the object. In this connection, reference is made to nth order absorption, if an absorption of m photons is effected by an atom or a molecule, leading to an n-fold electronic excitation. It has turned out in this case that the likelihood of such nth order absorption depends on the radiation intensity of the laser radiation.
In a transparent material, as it is present, in particular, in laser-surgical ophthalmic methods, several processes initiated by the laser radiation take place one after another in the case of non-linear interaction. If the power density of the radiation exceeds a threshold value, an optical breakthrough is produced in the transparent material, said breakthrough generating a plasma bubble in the material. Due to expanding gases, this plasma bubble grows after the optical breakthrough has formed. If the optical breakthrough is not maintained, for example because pulsed laser radiation has been used, the gas generated in the plasma bubble is re-absorbed by the surrounding material and the bubble disappears. If a plasma is generated at a material boundary surface (which may actually be located within a material structure as well), material removal is effected from said boundary surface. This is then referred to as photoablation, whereas in connection with a plasma bubble separating previously connected material layers one usually speaks of photodisruption. For the sake of simplicity, all such processes are summarized here by the term “optical breakthrough”, i.e. this term includes not only the actual optical breakthrough itself, but also the effects resulting therefrom in the material, caused by the non-linear interaction.
High localization of the effect of the laser radiation is required for high-precision machining. Collateral damage in adjacent regions of the material is to be avoided, if possible.
Looking at the probability of non-linear absorption, i.e. multi-photon absorption, it turns out that the probability of such interaction in a laser focus generated by a diffraction-limited optical system is proportional to the nth power of the quotient of laser power times the square of the numerical aperture and the square of the wave length used. For machining at maximum speed by means of non-linear absorption it is desired, of course, to optimize the likelihood of interactions. An increase in such likelihood of multi-photon absorption can be achieved by increasing the laser power, increasing the numerical aperture or decreasing the wave length of use of the laser radiation, as is directly evident from the above-explained context. Therefore, it has been indispensable hitherto in the prior art of non-linearly effective laser machining concepts to use optical systems having a high numerical aperture for focusing.
U.S. Pat. No. 5,894,916 as well as U.S. Pat. No. 6,110,166 describe methods for correcting visual deficiencies by means of suitably generating optical breakthroughs in the human cornea, so that the diffractive properties of the cornea are ultimately influenced in a selective manner. By sequential arrangement of the multiplicity of optical breakthroughs, a lens-shaped partial volume is isolated within the cornea of the eye, said volume then being removed from the cornea by means of a laterally opening cut. The shape of the partial volume is selected such that the diffractive or refractive properties of the cornea are changed after removal of the partial volume to achieve a desired correction of a visual deficiency.